I'm completing the renovation of an annex. The annex shares the mains supply with the main house which has a single phase connection rated as max 90A. I'd like to fit an electric shower (9.8kW) and an instantaneous electric water heater (9.5kW) into the annex; giving a combined draw of ~80A. Clearly using any other power hungry device in either the annex or main house (eg kettle, oven etc) would then take me over 90A. I thought that a possible solution is to only allow the instantaneous water heater to draw power if the shower isn't operating. Is anyone aware of any product which would cut power to the water heater if the shower is in use so that the max draw would be reduced or any other cunning solution (other than ripping out the shower and water heater)
What you need is a shower priority unit
Various types are available for two or three showers, with or without ladder or first-come-first-served priority. Made by Garo and available from larger electrical merchants in the UK and Ireland.
Why do you need two?
In Europe, some showers have their own, on-board, built-in on-demand water heater just for the shower. This is called an "electric shower".
Which begs the question of why you need two on-demand water heaters, since you are already installing one for the whole house. Simply using the one would solve the power problem.
Or, if your issue is that the shower and kitchen are distant from each other, and you don't want the long delay, then OK, we have to hunker down and have two heaters, and must solve the electrical problem. But then, again, why a heater dedicated to the shower? Why not a whole-bathroom heater so you can serve the sink also?
A relay would need to be current-activated
There's an easy electronics approach with a gadget called a reed relay, which placed next to a high current wire will engage. It needs 10 ampere-turns to operate, typically, and you'd have ~40A going through that wire, so merely orienting the reed relay correctly should get the job done. Essentially, drawing current on the shower heater would throw the relay and knock out the other heater (I presume you'd want it that way).
However, the challenge here is doing this with mains-rated equipment acceptable to your jurisdiction. It's not as simple as "raid the electronics parts bin" since the equipment must be mains rated.
Current-activated relays are available from industrial automation houses. They can be purchased with variable or fixed operating current setpoints (the current at which the relay contacts close). It is unlikely that you can find one that is capable of switching the full current of your load, so you will need a current activated relay controlling a single phase contactor that is rated for your full load.
You will need to find a current activated relay that has a DPST or DPDT relay at its output, because you will be wiring your contactor to the NC (normally closed) contacts of the relay. This will allow your water heater to run when there is no current flowing to your shower heater, and will disconnect your water heater when there is current flowing to your shower.
The current controlled relay, contactor, and 24VAC transformer (if necessary) will need to be mounted in an approved box. Do you know however, that if you are shower heater cycles on and off rapidly for whatever reason, this will cause the whole system to cycle rapidly, which may reduce the life of your other water heater.
I'm happy to add a schematic if you want to pursue this route.
A very simple manual solution is a dual-throw switch. These are often disguised under names like 3-way switch, interlock, generator transfer switch, selector switch, or changeover switch. The common terminal would be connected to the power source; the electric shower and the electric water heater would each connect to one of the switched terminals. A person could then select either of the two loads by throwing the switch. The solution could be embellished slightly by using a pair of relays to control the mains power. The relays could be chosen with mains-voltage coils or low voltage coils. In the latter case one might choose a 5 volt DC coil and power it with a USB charger adapter.
An automated solution could be built around relays with the addition of some control circuit. This could be as simple as a one-shot timer: push a button to select the electric shower; it remains energized for some fixed number of minutes and then power is automatically switched back to the electric water heater. Or the automatic control could be made more complex. For example both relays could normally be closed until a load is sensed (with a current transformer, a reed switch, a Hall effect sensor, etc) and the "other" relay could be switched off while the load remains active.
There are many manufacturers for these power switches, named priority/interlock/selector switch, shower change over switch, load shedding switch, Lastabwurfrelais etc., e.g. Garo, Eaton, ABB, Siemens, Eberle, Doepke, Hager.
Searching might be a problem because of these many names.
Another aspect is saving money and energy: A dedicated shower heater (tankless or with tank) can be combined with a counterflow heat exchanger. That way a huge amount of energy and money can be saved, some 25 to 45 % in practice. Some of the mentioned devices do only cut the low priority device if a certain (adjustable) current is measured, so some parallel use might be possible .
The electronic tank less heaters must be suited for the higher inlet water temperatures coming from the heat exchanger (20-30 degree Celsius), which might be restricted since the power electric elements like triacs or IGBTs are cooled by the incoming water.
There are many DIY instructions for simple inexpensive easy-to-build shower heat exchangers, f.e. https://m.youtube.com/watch?v=ZKrPT0_vSug
The use of a central electric water heater in combination with local water heaters can help to optimize the integration of modern systems like heat pumps, which rely on small temperature differences in order to get a high cooefficient of performance COP. This water may be warm enough for dish washing or hand washing, but for the shower an "afterburner" can be used.
So the use of heat exchangers with local electric water heaters is highly recommended - if not to say it should be mandatory.